The present invention pertains to the field of illumination devices for projection display systems (e.g. television, cinema, or data display), and more particularly to the application of two light sources having different optical characteristics in projection display systems using liquid crystal light valves.
In conventional projectors employing liquid crystal light modulators, a single lamp ordinarily provides the illumination. Typically the lamp consists of a high intensity discharge source in a simple conic reflector, i.e., a paraboloid or an ellipse usually with secondary refractive optics, for reasons of simplicity, cost, and size. The relative small effective xc3xa9ntendue, or optical extent, of the optics in recent projection systems restricts the amount of light which can be collected from the illuminator, and a discharge source is required in most cases to provide sufficient light. As the size of light modulators continues to decrease to reduce system cost, so does the limiting xc3xa9tendue of the optics, hence the more difficult it becomes to increase or maintain screen brightness with conventional illuminators. Conic section reflectors also have the disadvantage that the magnification of the arc image relayed to the projection optics varies with position of the aperture; further, the illumination is usually not uniform across the aperture as a consequence of the radiation patterns from the discharge source. The latter difficulty is usually addressed with the use of optical integrators such as fly-eye arrays, light-pipes, or fiber-optic bundles which superimpose these local intensity patterns at the image plane to provide relatively homogeneous illumination. The former difficulty reduces the light collection; while that may be improved somewhat with radical integrator designs, these may be difficult and expensive to manufacture. At present, light collection is the more pressing issue, but homogeneity is still important
Collection from single lamps may be improved by reflector design, and typically is implemented using one of two approaches. In the first, termed xe2x80x9cconstant magnificationxe2x80x9d, the reflector profile is modified from a parabola or ellipse in such a manner as to develop a constant image size across the aperture. This usually requires auxiliary optics, which may also help homogenize the intensity distribution at the output. See True, et al. and Shimizu. Secondly, one may devise more complex reflectors. True, et al. disclose a compound reflector having an elliptical section with a discharge source at the first focus. Part of the ellipse forward of the first focus is replaced with a spherical reflector segment, the light striking which is reflected back through the discharge tube to the elliptical reflector segment and thence to the second focus. In conjunction with an optional refractive corrector plate, one may utilize large focal lengths in relatively short reflectors. The collection from both of such systems is improved, but limited by the amount of light available from a single source. The constant magnification system of Shimizu, for example, has been shown to improve collection by between 12 and 20% in low-xc3xa9tendue systems over that using a high quality parabolic reflector, each using UHP (ultra-high pressure) short-arc discharge burners.
Multiple light beams may be applied to improve throughput, but since the xc3xa9tendue of each beam is summed together there may be less advantage that expected unless the xc3xa9tendue of the projector optics is large enough to make use of the input. Only for large system xc3xa9tendue will the multiplication of collection be close to the number of sources. For example, if two beams each of area A are superimposed on an aperture also of area A by some optical means, this can only be done if the divergence angle of the resulting beam is doubled; if it is necessary to maintain a small divergence angle, the beams can be combined only in an area approaching 2A.
Techniques for spatial superposition of light beams using reflective and refractive elements to improve projector brightness have been disclosed by, for example, Kokai 5-19355 and 6-242397 and EP683425.
There is the possibility, however, of combining the output of two sources such that the beams do not superimpose, but are principally complementary in space. This approach has the advantages of simplicity, compactness, and low cost since it can be done with inexpensive reflectors and no refractive optical components. It has the additional benefit of providing a better measure of illumination homogeneity across the input aperture of the optical system than that produced by conventional conic-section reflectors.
Accordingly, the present invention increases the quantity and quality of light beam directed through an aperture of a liquid crystal display projector over the prior art through the use of an apparatus comprising: orthogonally-arranged dual light sources having inexpensive geometric reflectors; and a fixed mirror for combining the beams from the two light sources. A first light source having a spherical rear reflector and a parabolic front reflector produces a light pattern having an annulus (e.g. donut-shaped) at the aperture. A second light source having a parabolic rear reflector and a spherical front reflector produces a complementary circular light pattern which illuminates the dark center of said annulus. Said mirror, having an elliptical shape, is positioned at a 45xc2x0 angle to the optical axis and centered on both light beams, producing a highly efficient and more uniformly distributed light source.
Another object of the present invention is to provide better beam quality through the use of complimentary beams rather than superimposed beams.
Another object of the present invention is to maximize the amount of light that can be directed through the aperture using the two light source apparatus.